Thesis title: Population dynamics of Tilia L. in Europe since the last interglacial period
Aim Reconstruction of dynamics and distribution patterns of the genus Tilia in Europe from the Eemian to present in order to project conservation interventions.
Location Europe and neighboring areas.
Methods A palynological survey based on pollen percentages with a threshold value of 0.5%, has been used. Tilia L. has been chosen as a model taxon because of its poorly-dispersed pollen grains, so that linden pollen samples substantially coincide with tree occurrence. Data from 497 pollen sites for the Eemian and the last glacial period, and 1961 for the Late Glacial and the Holocene, taken from online databases and from current literature papers, were collected, processed, geolocalized. Calibrated radiocarbon dates within the last 15 kyr-interval were used to construct maps at 1000-year intervals, whereas records belonging to the Eemian and to the early stages of the last glacial period beyond the reach of radiocarbon dating were stratigraphically set, thus leading to the production of separate maps for the Eemian and for the phases corresponding to MIS 5c, MIS 5b, MIS 5a, MIS 4, MIS 3, and MIS 2.
Results Distribution maps show a series of pollen sites scattered over most of Europe and its surroundings. It appears that during the Eemian the highest amounts of Tilia were in Poland and adjacent regions, whereas during the Last Glacial Maximum (LGM), in the Late Glacial and in the Holocene, lime was mainly concentrated in France, in Switzerland and in southern Germany. Its postglacial expansion over Europe occurred until 7 kyr, then Tilia underwent a progressive reduction – more marked since 4 kyr – in terms of percentage representation whilst its distribution range also at present remains nearly unchanged since this taxon did not disappear from any region.
Main conclusions This research work brought us to draw a series of conclusions:
- Tilia already occurred in Europe before the Eemian interglacial;
- Its distribution pattern reflects climate change, strongly influenced by glacial/interglacial cycles;
- Hotspots of lime spread were Poland in the Eemian and France/Switzerland surroundings in the Late Glacial/Holocene;
- During the Eemian, linden was very abundant in regions where it is very sparse at present probably because of a warmer climate than at present;
- Glacial refugia were mainly located in Greece, southern and central Italy, and the Alps;
- The Eemian distribution range of Tilia seems to anticipate its present-time dispersion;
- Not only anthropic factors such as global warming and forest clearance are responsible for reduced lime amounts in the last few millennia but also natural causes such as climate changes independent of man’s action or biotic factors such as competition.
My research, whose innovatory feature lies in collecting all data beginning from the Eemian up to present in an organic and orderly database – a kind of work never formerly carried out on lime and very useful to detect refuge areas - enabled us to infer that Eemian climate was less continental that present one, with winter temperatures warmer in the centre and the north-east of Europe than in the west and the north-west, due to the more westerly position of North Atlantic Stream and to the connection of the Baltic Sea both with the North Sea and with the Baltic Sea. However further efforts ought to be made in field work in order to provide pollen data for less studied regions, especially for Turkey which is hypothesized to be a glacial refuge for Tilia. This work may have practical developments in lime woodland reforestation plans, preferably integrated by predictive models.